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pH Scale

pH Scale. pH scale. Logarithmic scale expressing the H + concentration, [H + ]. If the pH changes by a factor of 1, the [H + ] changes by a factor of 10 . pH = -log[H + ]. -. Water & Self-Ionization. H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq)

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pH Scale

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  1. pH Scale

  2. pH scale • Logarithmic scale expressing the H+ concentration, [H+]. • If the pH changes by a factor of 1, the [H+] changes by a factor of 10. • pH = -log[H+]

  3. - Water & Self-Ionization • H2O (l) + H2O (l) H3O+ (aq) + OH- (aq) • H3O+ is called hydronium ion • OH-is called hydroxide ion • In reality, H+ and H3O+ are used interchangeably. • H+ is called a proton or a hydrogen ion.

  4. pH of Water • Water ionizes to produce both the hydronium ion and the hydroxide ion in equal amounts. • Ionization happens to a very small extent • [H+] = [OH-] = 1 X 10-7 M • At this concentration: pH = -log[H+] = 7

  5. pH to Molarity • pH = -log[H+]. Solve for [H+]. • -pH = log[H+] • Antilog(-pH) = [H+] • Say pH = 5. Then –pH = -5. • Antilog(-5) = 10-5. • The –pH becomes the power of 10!

  6. If the pH = 4 Antilog(-pH) = [H+] Antilog(-4) = 10-4 • The [H+] = ? • The pOH = ? • The [OH-] = ? 10 Antilog(-10) = [OH-] 10-10 = [OH-]

  7. Difference in pH = 2, which means the [H+] differ by 102 or 100. The pH of 5 solution has an [H+] that is 100 X greater than that of the pH 7 solution. • If the pH changes from 5 to 7, how do the [H+] compare?

  8. Significance of pH differences • Difference in pH =5 and pH =7 • Difference is 2 units of pH • The [H+] differ by 102 or 100 • The pH of 5 solution has an [H+] that is 100 X greater than that of the pH 7 solution.

  9. pH

  10. Ion Product Constant for Water • Value of the equilibrium constant expression for the self-ionization of water. H2O (l) + H2O (l) H3O+ (aq) + OH- (aq) Keq= [H+] [OH-] [H2O] Kw= [H+] [OH-] = 10-14

  11. If the pH = 2 The [H+] = ? 1 x 10-2M

  12. If the [H+] = 1 X 10-5 M 5 • The pH = ? PROBLEM: Relative acid strength How many time stronger is an acidic solution with A pH of 2 than an acidic solution with a pH of 5? 1000x

  13. pOH • By analogy, pOH is defined as –log[OH-] • Express [OH-] in scientific notation • If [OH-] = 1.0 X 10-3 M • Then pOH = -log(10-3) = -(-3) = 3.

  14. If the [OH-] = 1 X 10-3 pOH = -log[OH-] = -log(10-3) = --3 = 3 • The pOH = ? • The pH = ? • The [H+] = ? 14 – 3 = 11 Antilog(-11) = 10-11 M

  15. If the [H+] = 1 X 10-5 M 5 • The pH = ? • The pOH = ? • The [OH-] = ? 14 – 5 = 9 Antilog(-9) = 10-9

  16. pH + pOH = 14 11 • pH = 3, pOH = • pH = 7, pOH = • pH = 10, pOH = • pH = 9, pOH = • pH = 1, pOH = 7 4 5 13

  17. Acid, Base, or Neutral? • All H2O contains some H+ and some OH- • Pure H2O – concentrations are very low. • [H+] = [OH-] = 1 X 10-7 Molar. • When [H+] = [OH-], solution is neutral • Acidic solution: H+ > OH- • Basic solution: OH- > H+

  18. 10-3 A 10-11 11 9 5 B 10-9 A 12 2 10-12 M B 10-6 M 10-8 M 8

  19. How to safely test pH • Instrumental • use a pH meter • Indicators • use a series of indicators • Reaction • Acid if reacts with metals (other than Cu, Ag, or Au)

  20. Indicator • A substance that changes color over a narrow pH range. • Use several indicators to narrow down the pH range of the substance.

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